March 2, 1896.] 



KNOWLEDGE 



63 



greater than that of the transversal vibrations, and maybe 

 spoken of as infinitely great. If the existence of such 

 vibrations can be proved, and experimental evidence ob- 

 tained as to the behaviour and efl'ect of these longitudinal 

 vibrations, a great step foiward in our knowledge of the 

 ether may be about to be taken. When it is remembered 

 that the explanation of the cause of gravitation must be 

 in some way bound up with the nature of the ether, it 

 is easy to understand with what keen interest further 

 developments of Professor Eontgen's work are looked 

 forward to by all interested in the progress of science. 



Note. — The photogi-aph of a human foot was produced under the 

 direction of J. A. C. Porter, Esq., and is published as a lantern slide 

 by Messra. Leo Atkinson, of Greenwich, by whom it was kind)y lent 

 to us for reproduction. 



♦ 



In eclipses of the moon the brightness of the surface of 

 the moon within the shadow of the earth depends upon 

 the refraction and absorption of the sun's rays in passing 

 through the earth's atmosphere. I. vonllepperger calculates 

 the refractions and absorptions for various heights above 

 the earth's surface, and from these are deduced the ratios 

 between the eclipsed and ordinary luminosities for the 

 various portions of the shadow. The estimation of the 

 density of the atmosphere at various heights is based upon 

 difierent theories and the facts as to the incandeocence of 

 meteorites. One conclusion reached is that at the 

 beginning and end of totality the element of the moon's 

 disc furthest from the centre of shadow is two or three 

 hundred times as bright as the centre. As to the apparent 

 excess of the size of the umbra beyond the geometrical 

 shadow, this seems to be a physiological effect due to the 

 fact that the eye cannot detect minute differences of 

 brightness below a certain minimum of intensity. 



Hettfrs. 



[The Editors do not hold themselves responsible for the opinions or 

 statements of correspondents.] 



COilETS. 

 To the Editors of Knowledge. 

 Sirs, — Mr. Plummer's interesting article on short 

 period comets, and his mention of Mr. J. R. Hind in 

 connection with them, recalls to my mind an incident in 

 my early astronomical life which may be of interest, 

 considering how recently Mr. Hind has been lost to us. On 

 October 8th, 1858, I had the good fortune to be taken by 

 a mutual friend to Mr. Bishop's observatory in Regent's 

 PB,rk, for the purpose of seeing Doneti's celebrated comet, 

 then just passing its prime. Mr. Hind was of course 

 there, showing the comet to the visitors. In the interval 

 between the turns I got into conversation with the father 

 of Mr. J. R. Hmd, who told me that his son had had a 

 taste for astronomy from the time when he was quite a small 

 child, and that he was only twelve when he began regular 

 work as an astronomer with an instrument which he (the 

 father) had presented to his boy. I fear I have forgotten 

 the immediate results of this boyish start, as stated to me, 

 but the ultimate results are known throughout the world ; 

 for not only did Mr. Hind possess in a somewhat remark- 

 able degree the gift of explaining in singularly terse and 

 clear English any astronomical matter to which he applied 

 his pen, but his letters to the lunis through a period of 

 fully forty years on astronomical matters in general and on 

 comets in particular, furnished the general public with 

 astronomical news up to date when writers on our science 

 who appealed to the general press were few in number. 



G. F. Chambeks, F.E.A.S. 



MIEA CETI. 



To the Editors of Knowi.edge. 



Sirs, — After reading the discussion on Mira Ceti in your 

 columns, I resolved to watch this star closely during the 

 winter and spring months. Up to the end of 1895 it was 

 invisible to the naked eye, and during the first two weeks 

 of January cloudy weather prevented all observation. 



Judge, then, of my astonishment when on the 15th inst. 

 I perceived Mira to be of the 3i magnitude, quite con- 

 spicuous to the naked eye ! 



On the 19th it had increased to 3rd magnitude, and on 

 the 23rd, with the moon nearly nine days old just above 

 it, was easily ^asible as 3rd magnitude. On all these 

 occasions Mira appeared, as seen either with glasses or 

 through a telescope, of an orange colour. I should think 

 that this sudden rise of this truly wonderful star is 

 altogether unprecedented. 



I ought to add that the only observation I know of pre- 

 vious to mine in January was made by Miss E. Brown (see 

 Emjlish Mechanic, January 10th), who on January 2nd saw 

 Mira as a 6th magnitude star or brighter, and previous to 

 that, on December 15th, as 7th or 7i magnitude, while Mr. 

 David Flanery (see Emilish Mechatiic, January 10th) saw 

 it as 7^ magnitude on December 17th. It would be very 

 interesting to hear of any observations between January 

 2nd and -Januaiy 15th. Ivo. F. H. C.vrr-Gregg. 



PENT AC LE PUZZLE. 

 To the Editors of Knowledge. 



Sirs, — A short time ago I sent a problem to your 

 readers for solution, and now another equally interesting 



one has for some time occu- 

 pied my attention. Perhaps 

 some of your readers 

 would demonstrate how 

 many solutions can be given 

 of it. There seems but one 

 to me, but I cannot help 

 thinking there must be five 

 or more. The problem is to 

 place the numbers 1 to 10 in 

 the angles of a peutacle, as 

 marked in the accompanying 

 figure, so that each of the 

 five sides shall count one and 

 same number, and the five internal angles the same number, 

 and the five external angles double the same number. 



I. G. OUSELET. 



GEOGRAPHY AS A SCIENCE. 

 To tlie Editors of Knowledge. 

 Sirs, — The views which Dr. Mill has expressed on the 

 nature of geography, in his paper on " Geography as a 

 Science in England," in the January number of Knowledge, 

 are similar to those which I have long held and which I 

 have long known him to hold. But I an: much struck by 

 the freshness and vigour with which he has stated them. 

 His practical suggestion of a geographical description to 

 accompany the Ordnance Survey is also well worthy of 

 consideration. The general ide;i of a careful geographical 

 description of Britain is not new to me, but I frankly 

 confess that I feel much nearer to it now that, in 

 connection with the Ordnance map, it has taken such 

 precise form in Dr. Mill's mind. One point only strikes 

 me as requiring great caution. There is danger lest the 

 band of workers contemplated should approach this task 

 from various standpoints, many of them not geographical. 

 Both English and German experience shows the likelihood 



